The world’s largest living species, native to California's Sierra Nevada, faces a two-pronged risk from declining snowpack and rising temperatures. The threat to sequoias mirrors a growing danger to trees worldwide, with some scientists saying rapid warming this century could wipe out many of the planet's old trees.

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Few living things seem as permanent as the giant sequoia trees of California's Sierra Nevada. The largest species of flora or fauna on Earth, these towering redwood trees have held sway for millions of years in a narrow band of their native mountain habitat. With heights reaching 300 feet and girths as large as 150 feet, some sequoias can live in excess of 3,000 years before being naturally toppled by a combination of weather and gravity.

Although giant sequoias (Sequoiadendron giganteum) have survived previous eras of climate variability, human-caused climate change has so far not been their nemesis. But U.S. government and university researchers say the long-term existence of these trees could be threatened by the vagaries of a changing Sierra Nevada mountain snowpack and global warming. This combination could make it difficult for giant sequoias, particularly seedlings and young trees, to survive because they would be left with insufficient water to endure longer and warmer summers.

Nate Stephenson, a research ecologist with the U.S. Geological Survey (USGS), based near Sequoia & Kings Canyon National Parks, says that if climate warming continues as projected, tens of thousands of these ancient trees will be at risk in the coming century from destruction by either drought or climate-induced pathogens.

"In 25 years, we would see trouble for sequoia seedlings, then in 50 years trouble for the whole population," Stephenson said in an interview. "And in 100 years time, we could lose most of the big sequoias."